CMSPASHIN22004  
Dependence of twoparticle azimuthal correlations on the forward rapidity gap width in pPb collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}= $ 8.16 TeV  
CMS Collaboration  
3 June 2024  
Abstract: Measurements of the Fourier coefficients ($ V_{n\Delta} $) of the azimuthal distributions of charged hadron pairs produced in pPb collisions at 8.16 TeV are presented as functions of the forward rapidity gap width ($ \Delta\eta^{\rm{F}} $) for events in which particle production is concentrated in the direction of the outgoing Pb beam. The sample includes contributions from pomeron and photon induced processes as well as nondiffractive events. The data correspond to a luminosity of 174.5 nb$^{1}$ recorded with the CMS detector in the LHC Run 2. The mean track multiplicity decreases by 53% from the bin 0 $ \leq\Delta\eta^{\rm{F}} < $ 0.5 to that with $ \Delta\eta^{\rm{F}} > $ 2.5. The dependence of the results on the track multiplicity and the transverse momentum of the charged particles is also studied for the most forward events ($ \Delta\eta^{\rm{F}} > $ 2.5). The measured values of $ V_{1\Delta} $ are negative and decrease with $ \Delta\eta^{\rm{F}} $, while those of $ V_{2\Delta} $ are positive for the $ \Delta\eta^{\rm{F}} $ bins $ [0.5,1) $, $ [1,1.5) $, $ [2,2.5) $, and are consistent with zero elsewhere. The measured values of the elliptic flow coefficient ($ v_2 $) is provided for the $ \Delta\eta^{\rm{F}} $ bins $ [0.5,1) $, $ [1,1.5) $, $ [2,2.5) $. In the other bins, the $ v_2 $ results are consistent with zero. In these bins, upper limits at 95% C.L. are determined, ranging up to 0.13 for the most inclusive category. The values of the upper limits are similar to the values of $ v_2 $ measured in $ \gamma $p collisions with comparable multiplicity.  
Links: CDS record (PDF) ; CADI line (restricted) ; 
Figures  
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Figure 1:
Sketch of a singlediffractive pPb event (left). The interaction proceeds via the exchange of a pomeron ($\text{ IP} $); the proton remains intact or dissociates into a lowmass state, and the nucleus breaks up (X). A singlediffractive event as seen in the detector (right). Activity is observed only in one side of the detector and a large forward rapidity gap ($ {\Delta\eta^{\rm{F}} } $) is present. Both diagrams are extracted from [47]. 
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Figure 1a:
Sketch of a singlediffractive pPb event (left). The interaction proceeds via the exchange of a pomeron ($\text{ IP} $); the proton remains intact or dissociates into a lowmass state, and the nucleus breaks up (X). A singlediffractive event as seen in the detector (right). Activity is observed only in one side of the detector and a large forward rapidity gap ($ {\Delta\eta^{\rm{F}} } $) is present. Both diagrams are extracted from [47]. 
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Figure 1b:
Sketch of a singlediffractive pPb event (left). The interaction proceeds via the exchange of a pomeron ($\text{ IP} $); the proton remains intact or dissociates into a lowmass state, and the nucleus breaks up (X). A singlediffractive event as seen in the detector (right). Activity is observed only in one side of the detector and a large forward rapidity gap ($ {\Delta\eta^{\rm{F}} } $) is present. Both diagrams are extracted from [47]. 
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Figure 2:
$ N_{\mathrm{trk}}^{\mathrm{offline}} $ spectra for inclusive pPb events (labeled pPb, from Ref. [45]) and events from the nominal sample in different $ {\Delta\eta^{\rm{F}} } $ bins. 
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Figure 3:
$ N_{\mathrm{trk}}^{\mathrm{offline}} $ spectra for inclusive pPb events (labelled pPb, from Ref. [45]) and diffraction enhanced events. The distribution for the diffraction enhanced events (black data points) is the same as that shown in orange in Fig. 2 ($ {\Delta\eta^{\rm{F}} } > $ 2.5). Predictions based on EPOSLHC are also shown. The lower panel shows the ratio of data to MC yields. 
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Figure 4:
2D correlation distribution for multiplicity (left) and its projection on the $ {\Delta\phi} $ axis for $ {\Delta\eta} > $ 2, 2 $ \leq N_{\mathrm{trk}}^{\mathrm{offline}} < $ 40, and 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV (right). The onedimensional $ {\Delta\phi} $ distributions are symmetrized by construction around $ {\Delta\phi} = $ 0 and $ {\Delta\phi} = \pi $, so all the data are contained in [0, $ \pi $] and are averaged over $ {\Delta\eta} > $ 2. The Fourier coefficients $ {V_{n\Delta}} $ are fitted over the $ {\Delta\phi} $ range [0, $ \pi $]. 
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Figure 4a:
2D correlation distribution for multiplicity (left) and its projection on the $ {\Delta\phi} $ axis for $ {\Delta\eta} > $ 2, 2 $ \leq N_{\mathrm{trk}}^{\mathrm{offline}} < $ 40, and 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV (right). The onedimensional $ {\Delta\phi} $ distributions are symmetrized by construction around $ {\Delta\phi} = $ 0 and $ {\Delta\phi} = \pi $, so all the data are contained in [0, $ \pi $] and are averaged over $ {\Delta\eta} > $ 2. The Fourier coefficients $ {V_{n\Delta}} $ are fitted over the $ {\Delta\phi} $ range [0, $ \pi $]. 
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Figure 4b:
2D correlation distribution for multiplicity (left) and its projection on the $ {\Delta\phi} $ axis for $ {\Delta\eta} > $ 2, 2 $ \leq N_{\mathrm{trk}}^{\mathrm{offline}} < $ 40, and 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV (right). The onedimensional $ {\Delta\phi} $ distributions are symmetrized by construction around $ {\Delta\phi} = $ 0 and $ {\Delta\phi} = \pi $, so all the data are contained in [0, $ \pi $] and are averaged over $ {\Delta\eta} > $ 2. The Fourier coefficients $ {V_{n\Delta}} $ are fitted over the $ {\Delta\phi} $ range [0, $ \pi $]. 
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Figure 5:
Summary of measurements of $ V_{1\Delta} $ (left) and $ V_{2\Delta} $ (right) as a function of $ {\Delta\eta^{\rm{F}} } $. The results are for tracks with 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV. The $ {2} $ in the $ V_{n\Delta} $ variable name means that it was measured from the correlations from particle pairs. The filled circles indicate the data. The open triangles indicate the simulation, in blue for PYTHIA 8.3 and red for EPOSLHC. The data points are plotted at the center of the corresponding $ {\Delta\eta^{\rm{F}} } $ bin. The shaded bands indicate the size of the systematic uncertainties. The vertical lines indicate the size of the statistical uncertainties. 
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Figure 5a:
Summary of measurements of $ V_{1\Delta} $ (left) and $ V_{2\Delta} $ (right) as a function of $ {\Delta\eta^{\rm{F}} } $. The results are for tracks with 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV. The $ {2} $ in the $ V_{n\Delta} $ variable name means that it was measured from the correlations from particle pairs. The filled circles indicate the data. The open triangles indicate the simulation, in blue for PYTHIA 8.3 and red for EPOSLHC. The data points are plotted at the center of the corresponding $ {\Delta\eta^{\rm{F}} } $ bin. The shaded bands indicate the size of the systematic uncertainties. The vertical lines indicate the size of the statistical uncertainties. 
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Figure 5b:
Summary of measurements of $ V_{1\Delta} $ (left) and $ V_{2\Delta} $ (right) as a function of $ {\Delta\eta^{\rm{F}} } $. The results are for tracks with 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV. The $ {2} $ in the $ V_{n\Delta} $ variable name means that it was measured from the correlations from particle pairs. The filled circles indicate the data. The open triangles indicate the simulation, in blue for PYTHIA 8.3 and red for EPOSLHC. The data points are plotted at the center of the corresponding $ {\Delta\eta^{\rm{F}} } $ bin. The shaded bands indicate the size of the systematic uncertainties. The vertical lines indicate the size of the statistical uncertainties. 
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Figure 6:
Summary of measurements of $ V_{1\Delta} $ (left) and $ V_{2\Delta} $ (right) for the diffraction enhanced events for different classes of $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $. The results are for tracks with 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV. The $ {2} $ in the $ V_{n\Delta} $ variable name means that it was measured from the correlations from particle pairs. The filled circles indicate the data. The open triangles indicate the simulation, in blue for PYTHIA 8.3 and red for EPOSLHC. The data points are plotted at the mean of $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $ within the corresponding $ N_{\mathrm{trk}}^{\mathrm{offline}} $ bin. The shaded bands indicate the size of the systematic uncertainties. The vertical lines indicate the size of the statistical uncertainties. The inclusive pPb and $ \gamma $p results from Ref. [45] are also shown. 
png pdf 
Figure 6a:
Summary of measurements of $ V_{1\Delta} $ (left) and $ V_{2\Delta} $ (right) for the diffraction enhanced events for different classes of $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $. The results are for tracks with 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV. The $ {2} $ in the $ V_{n\Delta} $ variable name means that it was measured from the correlations from particle pairs. The filled circles indicate the data. The open triangles indicate the simulation, in blue for PYTHIA 8.3 and red for EPOSLHC. The data points are plotted at the mean of $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $ within the corresponding $ N_{\mathrm{trk}}^{\mathrm{offline}} $ bin. The shaded bands indicate the size of the systematic uncertainties. The vertical lines indicate the size of the statistical uncertainties. The inclusive pPb and $ \gamma $p results from Ref. [45] are also shown. 
png pdf 
Figure 6b:
Summary of measurements of $ V_{1\Delta} $ (left) and $ V_{2\Delta} $ (right) for the diffraction enhanced events for different classes of $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $. The results are for tracks with 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV. The $ {2} $ in the $ V_{n\Delta} $ variable name means that it was measured from the correlations from particle pairs. The filled circles indicate the data. The open triangles indicate the simulation, in blue for PYTHIA 8.3 and red for EPOSLHC. The data points are plotted at the mean of $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $ within the corresponding $ N_{\mathrm{trk}}^{\mathrm{offline}} $ bin. The shaded bands indicate the size of the systematic uncertainties. The vertical lines indicate the size of the statistical uncertainties. The inclusive pPb and $ \gamma $p results from Ref. [45] are also shown. 
Tables  
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Table 1:
Measured values of $ V_{n\Delta} $ as a function of $ {\Delta\eta^{\rm{F}} } $. The uncertainties include both statistical and systematic components. The table shows the $ v_2 $ results when they are nonzero. For the rest of the cases, upper limits at 95% C.L. are displayed with the $ \leq $ symbol. 
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Table 2:
Measured values of $ V_{n\Delta} $ for the diffractiveenhanced sample as a function of $ \langle N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} \rangle $ and $ N_{\mathrm{trk}}^{\mathrm{offline}} $ for both $ \mathrm{p^{trig}_T} $ bins. The uncertainties include both statistical and systematic components. The table also shows the 95% C.L. upper limits on $ v_2 $ displayed with the $ \leq $ symbol. In the lower table, $ v_2 $ values for $ \gamma $p events for 0.3 $ < \mathrm{p^{trig}_T} < $ 3 GeV [45] in similar $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $ ranges are given. 
Summary 
Longrange azimuthal twoparticle correlations have been searched for in protonlead (pPb) events at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} $=8.16 TeV as a function of the forward rapidity gap width ($ {\Delta\eta^{\rm{F}} } $) and the charged particle multiplicity ($ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $). Events have been selected by requiring an asymmetric distribution of energy in the forward and backward calorimeters, and no energy deposition in the region 2.5 $ < \eta < $ 3, so that activity is only present in the direction of the outgoing Pb beam. A separate set of measurements with a selection that enhances the fraction of diffractive events has also been carried out by requiring $ {\Delta\eta^{\rm{F}} } > $ 2.5. The average track multiplicity is lower in the diffractive sample than for inclusive protonlead (pPb) events, but larger than for photonproton ($ \gamma $p) events. The slope of the multiplicity distribution becomes steeper with increasing $ {\Delta\eta^{\rm{F}} } $. The mean track multiplicity $ \langle N_{\mathrm{trk}}^{\mathrm{offline}} \rangle $ decreases by 53% when $ {\Delta\eta^{\rm{F}} } $ changes from 0 $ \leq{\Delta\eta^{\rm{F}} } < $ 0.5 to $ {\Delta\eta^{\rm{F}} } > $ 2.5. The twodimensional twoparticle correlation functions have been averaged over $ {\Delta\eta} > $ 2 and the resulting $ {\Delta\phi} $ distributions have been studied in terms of its Fourier coefficients $ V_{1\Delta} $ and $ V_{2\Delta} $. The measured values of $ V_{1\Delta} $ are negative and decrease with $ {\Delta\eta^{\rm{F}} } $, while those of $ V_{2\Delta} $ are positive for the $ {\Delta\eta^{\rm{F}} } $ bins $ [0.5,1) $, $ [1,1.5) $, $ [2,2.5) $, and are consistent with zero elsewhere. The results have been compared with the predictions of the PYTHIA 8.3 Monte Carlo simulation, which does not include collective effects, and with EPOSLHC simulation, which includes such effects. The measured values of the elliptic flow coefficient ($ v_2 $) are provided for the bins $ [0.5,1) $, $ [1,1.5) $, $ [2,2.5) $. In the other bins, the $ v_2 $ results are consistent with zero. In these bins, 95% confidence level upper limits have been determined; they are stable across the $ {\Delta\eta^{\rm{F}} } $ and $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $ regions studied, and range up to 0.13 for the most inclusive $ N_{\mathrm{trk}}^{\mathrm{offline}} $ bin. The values of the upper limits are similar to the values measured in $ \gamma $p events for comparable $ N_{\mathrm{ch}}^{\eta  < 2.4,p_{\rm{T}} > 0.3} $ and $ \mathrm{p^{trig}_T} $ ranges. 
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